Red Hat and Microsoft, about as virulent a pair of enemies as has ever existed, threw a leash over their fire-breathing dragons long enough on Monday to announce a cooperative pact forced on them by their mutual users - folks who may be holding back from committing to virtualization ahead of a truce.

The companies say they're going to support each other's operating systems as guests on each other's current virtualization schemes. Windows Server will be supported on Red Hat's transient Xen virtualization and Red Hat 5 will be supported on Microsoft's Hyper V widgetry.

What happens when Red Hat moves on to its own Qumranet-derived KVM hypervisor when it gets to RHEL 6.0 is unclear.

The way the détente works each company will join the other's virtualization validation/certification program and test and validate each other's operating systems on each other's hypervisors so customers can get the mixed virtualization support they want.

It will take until sometime in the second half for all the work to be done and for Red Hat to produce the necessary drivers. Microsoft will also have Linux physical and virtual management tools.

The key point for Red Hat is that this interoperability deal contains no embarrassing patent or open source license concessions, like the two-year-old Microsoft-Novell deal that plucked Novell's chestnuts out of the fire but made it something of a pariah with the open source set, a proviso that may or may not explain why it took the Microsoft and Red Hat eight months, as they admit, to negotiate the arrangement.

There are also no financial terms beyond the usual certification testing fees.

Microsoft's IP demands supposedly short circuited other Microsoft-Red Hat interoperability discussions. There's no IP sharing in this arrangement, according to Microsoft, so no patent licensing. It's not giving anything up.

Red Hat's VP of its platform business unit Scott Crenshaw calls the deal a "major step forward for the industry" while acknowledging that it is "rare that these two companies publicly work together."

Microsoft and Red Hat are supposed to represent 80% of today's virtualized operating systems according to IDC's numbers, which is what makes the truce important.

The deal will cover folks with valid support agreements or current RHEL subscriptions who will be able to go to either company for support. Otherwise they can arrange per-call support.

Red Hat has a deal with VMware that validates RHEL on ESX but not with Citrix, which competes with Red Hat on XEN. Microsoft currently supports VMware, Novell, Citrix and Cisco.